1. Field of the Invention
The present invention relates to a photovoltaic device (or a photovoltaic element) such as a solar cell and a process for the production thereof. More particularly, the present invention relates to a photovoltaic device (a photovoltaic element) having an electrode structure with improved durability and a process for the production thereof.
2. Related Background Art
In recent years, power generation means utilizing various kinds of natural energies have received public attention from the viewpoint of environmental protection. In particular, attention has been focused on a sunlight power generation system which generates electric power by irradiating sunlight to photovoltaic elements (or solar cells) without causing pollution.
However, such sunlight power generation system still have a subject necessary to be solved in order to make use thereof to be widespread such that the initial cost required for establishing a sunlight power generation system using photovoltaic elements (solar cells) is relatively high because photovoltaic elements are costly.
In order to overcome this subject, various kinds of photovoltaic elements (solar cells) and processes for the production thereof have been proposed up till now.
In particular, there is a proposal to reduce the cost required for establishing a sunlight power generation system by making each photovoltaic element (solar cell) used therein have a large light receiving area. That is, the output voltage of one photovoltaic element (solar cell) is as low as several volts and because of this, in order to achieve a high output voltage, it is necessitated that a plurality of photovoltaic elements (solar cells) are serialized. There are known several methods for serializing a plurality of photovoltaic elements (solar cells) in order to make it possible to reduce the cost of the sunlight power generation system. One of these methods is to diminish the number of photovoltaic elements (solar cells) serialized by enlarging the light receiving area of each photovoltaic element. Particularly, this method is intended to reduce the cost of the sunlight power generation system by enlarging the light receiving area of each of the photovoltaic elements used therein as large as possible to diminish the number of the photovoltaic elements to be serialized as well as the production process of the sunlight power generation system is simplified.
A feature of such large area photovoltaic element (solar cell) is to have an electrode structure on the light incident side in that a metallic wire is used.
FIG. 8 is a schematic view illustrating an example of a photovoltaic element (a solar cell) having a relatively large light receiving area. In FIG. 8, reference numeral 810 indicates a collecting electrode (or a grid electrode) comprising a plurality of metallic wires, 811 a bus bar electrode, 807 a photovoltaic layer, 808 a transparent electrode layer, and 812 a backside electrode.
In the photovoltaic element shown in FIG. 8, a current flows by way of a path connecting the backside electrode, photovoltaic layer, transparent electrode layer, collecting electrode, and bus bar electrode. In FIG. 8, the current flowing from the transparent electrode via the collecting electrode to the bus bar electrode is indicated by an arrow. As shown in FIG. 8, the current convergently flows from the transparent electrode to the collecting electrode, followed by flowing in the bus bar electrode. It is preferred for the metallic wires as the collecting electrode to be as thinner as possible so as to prevent light incident to the photovoltaic element from being shielded by the collecting electrode.
It is easily understood from FIG. 8 that as the light receiving area of the photovoltaic element is enlarged, each of the metallic wires as the collecting electrode is made to be longer accordingly, where the quantity of current flown therein is increased. As the quantity of current flown in the collecting electrode is increased, the Joule loss of the collecting electrode is increased, and because of this, it is necessary that the electrical resistance of the collecting electrode is reduced. In the case of a photovoltaic element having a size of 10 cm square, in general, a printed electrode obtained by printing a conductive resin on the transparent electrode layer and forming a low melting point metal such as a solder thereon by a reflow process is effective to use as the collecting electrode. However, in the case of a large area photovoltaic element whose size is larger than the above photovoltaic element, when such printed electrode is used as the collecting electrode, the electrical resistance of the collecting electrode unavoidably becomes higher to increase the Joule loss. In order avoid this situation, there is considered a manner to thicken the printed electrode.
However, this manner is not effective for the following reason. It is difficult to thicken the printed electrode in the longitudinal direction and therefore, the remaining solution is to thicken the printed electrode in the width direction. This entails a problem in that the printed electrode thickened in the width direction shields incident light, where the quantity of power generated is lowered. Accordingly, for a large area photovoltaic element, it is necessitated to use a collecting electrode comprising a plurality of metallic wires having a low electrical resistance which less shields the incident light. By the way, such metallic wire is thin in the width direction but is thicker than the printed electrode in the longitudinal direction. Therefore, the use of the metallic wire makes it possible to realize a collecting electrode which is thin and is low in electrical resistance.
U.S. Pat. No. 4,260,429 discloses a photovoltaic element in which a metallic wire is used the collecting electrode.
Besides, in order to establish an electrical connection of a collecting electrode comprising a metallic wire and a bus bar electrode in an electrode structure of a photovoltaic element, there are know a method of joining the metallic wire with the bus bar electrode by direct welding or fusing, and a method of joining the metallic wire with the bus bar electrode through a low melting metal represented by a solder or through a conductive resin.
Now, in the electrode structure of a photovoltaic element as disclosed in U.S. Pat. No. 4,260,429, the electrical connection portion between the metallic wire and the bus bar electrode is very important factor in view of the following factors.
1. The electrical connection portion is necessary to be low in electrical resistance. As above described, a current convergently flows in the collecting electrode, where a large current flows also in a portion of the electrical connection portion which located at the leading end of the collecting electrode. To sufficiently suppress the Joule loss generated by the flow of such a large current, the electrical resistance of the electrical connection portion is necessitated to be sufficiently low.
2. The electrical connection portion is necessary to be high in stress resistance. A photovoltaic element (a solar cell) is used in an outdoor environment. In this, although the photovoltaic element is used by configuring as a solar cell module, a relatively large stress is applied to the photovoltaic element itself due to wind or snow cover. The stress applied to the collecting electrode is liable to be concentrated at the electrical connection portion thereof with the bus bar electrode. Accordingly, the stress resistance of the electrical connection portion is necessitated to be sufficiently large.
3. The electrical connection portion is necessary to have sufficient durability so that the electrical connection portion is maintained in a stable state over a long period of time without deteriorating its low electrical resistance and large stress resistance.
Now, when calculation is made on the basis of the present power cost, it will take about ten and several years to compensate the initial cost of a conventional sunlight power generation system by the amount provided by the generated power energies. Accordingly, the durability of the photovoltaic element is required to be as long as twice or more that of any other general electronic elements. Further, since the sunlight power generation system is directly exposed to an outdoor environment, the durability thereof is actually required to be as long as about ten times the durability of any other general electronic elements.
In order for the electrical connection portion to satisfy the three performances described in the above 1 to 3 in good balance, it is preferred to be configured such that the metallic wire as the collecting electrode and the bus bar electrode are joined through a conductive resin, for the following reason. When the metallic wire and the bus bar electrode are joined by direct welding or fusing or they are joined through a low melting metal such as a solder, the rigidity of the joint (the electrical connection portion) between the metallic wire and the bus bar electrode becomes excessively high, so that stress is concentrated at a portion of the metallic wire situated in the vicinity of the joint (the electrical connection portion) to increase the possibility that the metallic wire is broken.
An example of the conventional joint structure (the conventional electrical connection structure) in that the metallic wire as the collecting electrode and the bus bar electrode are joined through a conductive resin is disclosed in Japanese Laid-open Patent Application No. Hei 8-236796. A cross-sectional structure of the electrical connection portion described in this document is typically as shown in FIG. 10.
As shown FIG. 10, a metallic wire 1001-1 (cross-section of the metallic wire) is electrically and mechanically joined with an electrode member 1002 through a joint composed of a resin 1003-2 and a number of conductive particles 1003-1. The concrete size of the joint structure is as follows: the thickness of the metallic wire is in a range of about several tens to several hundreds xcexcm; and the size of the conductive particle is about several tens nm if the conductive particle is made from carbon and is in a range of about several to ten and several xcexcm if the conductive particle is made from a metal. Further, the joint is concretely formed by uniformly dispersing conductive particles in a resin to form a conductive resin, disposing the conductive resin between the metallic wire and the electrode member, and hardening the conductive resin.
In the joint structure (the electrical connection structure) in this case, the mechanical strength of the joint portion composed of the conductive particles and the resin is relatively weak at an interface between the joint and the metallic wire, that is, the bonded plane between the conductive resin and the surface of the metallic wire and its neighborhood. Particularly, in the stress resistance test and stress resistance durability test, peeling or breakage is sometimes occurred at the bonded plane or its neighborhood. The occurrence of peeling or breakage at the bonded plane becomes a serious problem.
Thus, there is an increased demand for improving the electrical connection portion (the join portion) in the electrode structure of the photovoltaic element (the solar cell) so that it excels in stress resistance and durability.
The present invention is aimed at solving the foregoing problems in the prior art and achieving a photovoltaic element (a solar cell) having an improved electrode structure in which a collecting electrode comprising a metallic wire and a bus bar electrode are joined through a conductive resin to form an electrical connection between the collecting electrode and the bus bar electrode which has improved stress resistance and improved durability.
Another object of the present invention is to provide a photovoltaic element having an electrode structure in which the mechanical strength of a bonded plane between a collecting electrode comprising a metallic wire and a conductive resin and its neighborhood is improved.
A further object of the present invention is to provide a photovoltaic element having a first electrode comprising a metallic wire, a second electrode connected to said first electrode, and a joint portion composed of conductive particles and a resin arranged between said metallic wire as said first electrode and said second electrode, characterized in that a volume density of said conductive particles in said joint portion is decreased in a direction from the surface of the second electrode to the surface of the metallic wire as the first electrode, whereby the joint portion has improved stress resistance.
A further object of the present invention is to provide a process for the production of said photovoltaic element. The process includes following two embodiments.
A first embodiment of the production process is characterized by including a steps of disposing a resin (A) containing conductive particles dispersed therein at a desired volume density, and a step of stacking a resin (B) on said resin (A), said resin (B) containing said conductive particles dispersed therein at a desired volume density which is different from said volume density of the conductive particles dispersed in the resin (A), thereby forming aforesaid joint portion.
A second embodiment of the production process is characterized by including a steps of forming a mottled cover portion covering the surface of the metallic wire in a mottled pattern, and a step of disposing a resin containing conductive particles dispersed therein in such a manner that the resin is in contact with the mottled cover portion. In this process, preferably, the mottled cover portion is formed by disposing a resin containing a filler on the surface of the metallic wire, and irradiating the resin containing the filler with an energy beam.
The constitution, advantages, and preferred embodiments of the present invention will be detailed with reference to the drawings later.